Fiber-reinforced prepreg and composite material obtained therefrom

ABSTRACT

There are provided a prepreg obtained by impregnating into a fiber reinforcement a resin composition including 40 to 70 parts by weight of an aromatic bismaleimide (A) expressed by a specific general formula, 60 to 30 parts by weight of an alkenylphenol (B) expressed by a specific general formula, 1 to 10% by weight of a polyetherimide (C) of a thermoplastic resin base on the total amount of components (A) and (B), and 15 to 50% by weight of an amorphous polyimide (D) having a glass transition temperature of 200° C. or higher, and a composite material obtained by heating and curing the prepreg. Also provided are the fiber-reinforced prepreg and the composite material without spoiling thermal resistance characteristic of the aromatic bismaleimide resin used as a main component of a resin and having excellent toughness imparted thereto.

TECHNICAL FIELD

The present invention relates to a fiber-reinforced prepreg (or maysimply be called a prepreg) and a composite material obtained therefrom,and more particularly, to a fiber-reinforced prepreg to which excellenttoughness is imparted without spoiling the thermal resistance of anaromatic bismaleimide resin used as a main component of a matrix resin,or a composite material using the same.

BACKGROUND ART

Fiber-reinforced composite materials make the best use of the featuresof their excellent specific strength and specific elasticity and arewidely applied to applications such as aviation/space articles.Conventionally, epoxy resins are primarily used as a matrix resin;however, the epoxy resins also suffer from the problem of not being ableto sufficiently satisfy the requirement of thermal resistance against200° C. or higher. On the other hand, although polyimides known as ahigh-temperature resin are excellent in thermal resistance, they cause aproblem in formability and thus their practical use to a matrix resinare behind.

In these situations, bismaleimide resins such as aromaticbismaleimide-based resins excellent in balance between thermalresistance and moldability are paid to attention as a matrix resin forfiber-reinforced composite materials such as carbon fibers. However,bismaleimide resins have the defect of low toughness, and thereforetheir applications are considerably limited. Although a method ofblending a rubber component or a thermoplastic resin and a method ofcopolymerizing other monomers are proposed as a method of improving thisdefect of bismaleimide resins, the resin has created problems such asthe improvement of toughness being insufficient as compared to adecrease in physical properties such as thermal resistance being large.In addition, while a method of inserting a kind of adhesion layer or ashock absorbing layer, called an inter leaf into an interlayer isproposed, it has disadvantages that the fiber content is not increasedand handleability is poor, so that the method has not generally beenused.

Japanese Patent No. 3312441 (Patent Document 1) discloses a prepregexcellent in impact resistance properties in which thermoplastic resinparticulates are localized in a thermosetting resin composition.However, its Examples show the use of an epoxy resin as a matrix resinand do not disclose the advantage of aromatic bismaleimide resins.

Japanese Patent Application Laid-Open Publication No. 06-41332 (PatentDocument 2) discloses a prepreg in impact resistance properties that isproduced by using thermoplastic resin particulates having an imidelinkage or thermoplastic resin particulates having a silicone linkagefor an aromatic bismaleimide resin composition. However, the prepregdoes not have sufficient improved impact resistance properties and alsothe glass transition temperature and solvent-resistant properties (MEK)of the fiber-reinforced composite material are not necessarilysufficient.

-   Patent Document 1: Japanese Patent No. 3312441-   Patent Document 2: Japanese Patent Application Laid-Open Publication    No. 06-41332

Additionally, even in other documents (Nonpatent Document 1), it isconceptually and generally known that a thermosetting resin isreinforced by addition of thermoplastic resin particulates thereto. Forexample, it is reported that the addition of a polyetherimide of athermoplastic resin to a bismaleimide resin improves the toughness ofthe cured resin. However, since solvent resistance of polyetherimides(particularly, solvent resistance to methyl ethyl ketone) is poor, thesolvent-resistant properties of the bismaleimide resin are remarkablyand disadvantageously decreased when the amount of addition of apolyetherimide is increased. Moreover, when the amount of addition of apolyetherimide is small, the solvent-resistant properties of thebismaleimide resin are good, but there is also the problem that thecured resin shows merely little toughness improvement.

-   Nonpatent Document 1: 33rd International SAMPE Symposium, 1988

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The study of the inventors has shown that addition of severalthermoplastic resins to a bismaleimide resin lowers the toughness of thecured resin as compared with that of an unmodified resin in some cases.As such, it seemed to be almost impossible to predict the toughnessimprovement of a resin by the addition of wide kinds of thermoplasticresins in the bismaleimide resin system. However, the inventors havediligently studied and found that the use of a certain kind ofthermoplastic resin makes it possible to effectively improve thetoughness of a bismaleimide resin without spoiling thermal resistanceand solvent resistance. In addition, this bismaleimide resin-based resincomposition improved in toughness has been found to be very excellentfor a fiber-reinforced prepreg and a composite material.

An object of the present invention is to provide a fiber-reinforcedprepreg and a composite material which are used as main components of amatrix resin and to which excellent toughness is imparted withoutspoiling thermal resistance that is a characteristic of an aromaticbismaleimide resin.

Means for Solving the Problems

The present invention is a prepreg that is produced by impregnating aresin composition including components (A) to (D) below as essentialcomponents into a fiber reinforcement (invention of claim 1). Inaddition, another aspect of the invention is a composite materialproduced by heat-curing such prepreg, wherein component (D) is localizedbetween laminated layers and forms a phase separated structure(invention of claims 8).

(A) 40 to 70 parts by weight of an aromatic bismaleimide expressed byformula [1] below,(B) 60 to 30 parts by weight of an alkenylphenol expressed by formula[2] below,(C) 1 to 10% by weight of a polyetherimide of a thermoplastic resinbased on the total amount of components (A) and (B) above, and,(D) 15 to 50% by weight of an amorphous polyimide the glass transitiontemperature of which is 200° C. or higher based on the total amount ofcomponents (A) and (B) above.

wherein, X is —CH₂—, —C(CH₃)₂—, —SO₂ ⁻, —SO—, —CO—, —S— and —O—.

wherein, R₁ or R₂ is each independently an allyl group, n is an integerof 1 to 4, and Y is —CH₂—, or —C(CH₃)₂—

Advantages of the Invention

A prepreg and a composite material having as a matrix resin a resincomposition including an aromatic bismaleimide resin of the presentinvention have excellent toughness without spoiling the excellentthermal and mechanical characteristics of the matrix resin. Hence, aprepreg and a composite material of the present invention can besuitably used for structural materials for aircrafts and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ratio of component (A) to (B) of the resin composition of theinvention is 40 to 70 parts by weight: 60 to 30 parts by weight,preferably 50 to 60 parts by weight: 50 to 40 parts by weight. Inaddition, component (C) is 1 to 10% by weight, preferably 2 to 5% byweight, based on the total amount of components (A) and (B), andcomponent (D) is 15 to 50% by weight, preferably 20 to 30% by weight,based on the total amount of components (A) and (B). Other well-known,various resins and additives can optionally be added to the resincomposition of the invention within the scope of not losing the objectand advantages of the invention.

Component (D) of the invention is an amorphous polyimide having a glasstransition temperature of 200° C. or higher and, of these, preferably anamorphous polyimide having 40% by mole or more of a repeated structuralunit expressed by formula [3] below and 5 to 60% by mole of a repeatedstructural unit expressed by formula [4] below (invention of claim 2).

wherein, R₃ is a quadrivalent group selected from the group consistingof a monocyclic aromatic group, a condensed polycyclic aromatic groupand non-condensed polycyclic aromatic groups produced by linkingdirectly or via a cross-linked member an aromatic group to each other.

An aromatic bismaleimide of component (A) expressed by formula [1] aboveof the present invention can be obtained by a well-known method in whicha corresponding aromatic diamine reacts with maleic anhydride. Thearomatic bismaleimide is preferably dissolved in an alkenylphenolcomponent.

The aromatic bismaleimides can include N,N′-m-phenylene bismaleimide,N,N′-p-phenylene bismaleimide, N,N′-m-toluoylene bismaleimide,N,N′-4,4′-biphenylene bismaleimide,N,N′-4,4′-(3,3′-dimethylbiphenylene)bismaleimide,2,2-bis[4-(4-maleimidephenoxy)phenyl]propane, and the like.

Among the aromatic bismaleimides expressed by formula [1] above,N,N′-4,4′-diphenylmethane bismaleimide (bismaleimidediphenylmethane),N,N′-4,4′-diphenyl ether bismaleimide, N,N′-m-toluoylene bismaleimide,2,2-bis[4-(4-maleimidephenoxy)phenyl]propane,N,N′-4,4′-diphenylsulfonebismaleimide, N,N′-4,4′-benzophenonebismaleimide, and the like are preferred from the viewpoint of resinthermal resistance after curing. In particular,N,N′-4,4′-diphenylmethane bismaleimide, N,N′-4,4′-diphenyl etherbismaleimide, N,N′-m-toluoylene bismaleimide and2,2-bis[4-(4-maleimidephenoxy)phenyl]propane are preferred. The aromaticbismaleimides above may be used alone or in combination of two or morekinds.

The alkenylphenols of component (B) expressed by formula [2] of theinvention include O,O′-diallylbisphenol A,4,4′-dihydroxy-3,3′-diallyldiphenyl,bis(4-hydroxy-3-allylphenyl)methane,2,2′-bis(4-hydroxy-3,5-diallylphenyl)propane, 2,2′-diallylbisphenol F,4,4′-dihydroxy-3,3′-diallyldiphenyl ether, and the like.

Among them, O,O′-diallylbisphenol A,2,2′-bis(4-hydroxy-3,5-diallylphenyl)propane, 2,2′-diallylbisphenol F,and the like are preferred because the resin after curing has a highglass transition temperature, and O,O′-diallylbisphenol A isparticularly preferred since it makes the viscosity of the resin priorto curing low.

One example of particularly preferred combinations of aromaticbismaleimides and alkenylphenols, as indicated above, can include the“Matrimid 5292” series (made by Huntsman Corp.) commercially availablefrom Huntsman Corp., namely, 4,4′-bismaleimide diphenylmethane (Matrimid5292A) and O,O′-diallylbisphenol A (Matrimid 5292B).

The polyetherimide of thermoplastic resin of component (C) above of theinvention is used in an amount of 1 to 10 parts by weight based on 100parts by weight of the sum of components (A) and (B) (1 to 10% by weightbased on the total amount of both components), preferably in an amountof 2 to 5 parts by weight. If exceeding 10 parts by weight, the amountis improper since the solvent-resistant properties are worsened. Apolyetherimide having a repeated structural unit expressed by formula[5] below and an number average molecular weight of 3,000 to 50,000 ispreferred as the polyetherimide of thermoplastic resin of component C.

In particular, the polyetherimide “Ultem 1000” (made by GE Corp.)commercially available from GE Corp is preferred. It is preferred thatthis polyetherimide is completely dissolved in a resin componentincluding 40 to 70 parts by weight of an aromatic bismaleimide, i.e.,component (A) and 60 to 30 parts by weight of an alkenylphenol, i.e.,component (B) and used.

The amorphous polyimide that has a glass transition temperature of 200°C. or higher, preferably 220° C. or higher, and is component (D) used inthe invention, preferably includes a polyimide having 40% by mole of therepeated structural unit expressed by formula [3] above and 5 to 60% bymole of the repeated structural unit expressed by formula [4] above, andparticularly “AURUM PD450M” (made by Mitsui Chemicals, Inc.)commercially available from Mitsui Chemicals, Inc.

The particulation of this amorphous polyimide is made by pulverizationand classification by means of a pulverizer. Particulation can uniformlydisperse polyetherimide (C) of the thermoplastic resin in a mixture ofan aromatic bismaleimide (A) and an alkenylphenol (B). In particular,the particulates having a particle diameter of 100 μm or less arepreferred. (invention of claim 3) More preferable is a particle diameterof 1 to 20 μm. The case where the particle diameter exceeds 100 μmcauses concentration unevenness, etc. during compatibilization at a hightemperature, and thus there is the possibility to pose the problem oflowering the strength of cured resin.

The amount of addition of an amorphous polyimide as a resin component ofthe invention, e.g., AURUM PD450M (made by Mitsui Chemicals, Inc.), is15 to 50 parts by weight based on 100 parts by weight of a mixture of anaromatic bismaleimide and an alkenylphenol (15 to 50% by weight based onthe total amount of the mixture). The amount is more preferably from 20to 30 parts by weight. The reason why the amount of addition of anamorphous polyimide is set to be in the range is that, when the amountof addition is 50 parts or more, the viscosity of the resin compositionis increased, thereby posing the problem of spoiling the tack propertiesof a prepreg that uses the present resin composition. In addition, whenthe amount of addition of an amorphous polyimide is 15 parts by weightor less, the improved effect of impact resistance properties isinsufficient.

The prepreg of the invention can be produced, for example, byimpregnating into a fiber reinforcement a dispersion resin compositionobtained by uniformly dispersing amorphous polyimide particulates ofcomponent (D) in a uniform resin composition obtained by completelydissolving in components (A) and (B) a polyetherimide of thermoplasticresin of component (C). The means and the method of impregnation are notparticularly limited and the methods may include a method ofimpregnating a fiber reinforcement into a solution or a dispersionliquid of a resin composition, a method of laminating a film of a resinor composition on a fiber reinforcement and pressurizing and heating itby a heated roll and then impregnating the melted resin component into afiber reinforcement, and the like.

When the particle diameter of amorphous polyimide particulates ofcomponent (D) is larger than the diameter of single fibers of the fiberreinforcement, the amorphous polyimide particulates of component (D) isfiltered by the fiber reinforcement in a prepreg impregnation step, anda prepreg in which the polyimide particulates are localized anddistributed in the vicinity of the surface of the prepreg is obtained(invention of claim 4).

Amorphous polyimide particulates of component (D) may be activelylocalized in the surface vicinity of one or both sides of a prepreg. Afiber reinforcement is laminated on a film of a resin composition madeof components (A), (B) and (C) and the resulting material is heated andpressurized by a heated roll or the like to thereby produce a prepreg.Amorphous polyimide particulates of component (D) are sprinkled on oneor both sides of this prepreg and the resulting material is pressurizedand heated by a heated roll to thereby obtain a composite materialhaving localized thereon the amorphous polyimide particulates.

Further, the amorphous polyimide of component (D) can be used as afibrous nonwoven fabric. For example, a fiber reinforcement is laminatedon a film of a resin composition made of components (A), (B) and (C), ora fiber reinforcement is sandwiched with the films, and a fibrousnonwoven fabric of an amorphous polyimide is inserted into between thefiber reinforcement and the film, and then the resulting material isheated and pressurized by heated roll or the like to thereby obtain aprepreg in which the fibrous nonwoven fabric is placed in the surfacevicinity of one or both sides of a prepreg (invention of claim 5).

Particulates and a fibrous nonwoven fabric may be used together as anamorphous polyimide of component (D). In such case, a prepreg isobtained in which particulates and a fibrous nonwoven fabric are placedin the surface vicinity of one or both sides of a prepreg (invention ofclaim 6). For example, a film of a dispersion resin composition obtainedby uniformly dispersing amorphous polyimide particulates of component(D) and a fiber reinforcement are laminated on resin composition made ofcomponents (A), (B) and (C), or a fiber reinforcement is sandwiched withthe films, and further a fibrous nonwoven fabric of an amorphouspolyimide is inserted into between the fiber reinforcement and the film,and then the resulting material is heated and pressurized by heated rollor the like to thereby obtain a prepreg in which particulates and thefibrous nonwoven fabric are placed in the surface vicinity of one orboth sides of the prepreg.

A prepreg used in the present invention may be any prepreg and is notparticularly limited. A prepreg refers to a molding intermediatematerial the handleability of which is made good by impregnating amatrix resin into a fiber reinforcement and removing flowability andadhesiveness. In the present invention, the form of a fiberreinforcement that forms a prepreg is not particularly limited. Usually,in addition to, for example, a material including the warp and the weftsuch as a plain fabric, twill fabric and satin fabric, a fiberreinforcement is used in forms such as a uniaxial fabric made byarranging a fiber bundle in one direction to form a sheet shape andstitching it with a stitch thread in the perpendicular direction, amultiaxial woven fabric made by laminating a plurality of sheetmaterials stretched in one direction by changing their angles andstitching them with a stitch thread in the perpendicular direction, andthe like. Alternately, a material made by arranging a fiber bundle(strand) in parallel in one direction to form a sheet shape andimpregnating a resin thereinto, a tape-like prepreg (bias tape prepreg)made by arranging fiber bundles in ±45° and impregnating a resin intoit, or the like may be allowable. The content of a resin in a prepreg istypically in the range of 20 to 80% by weight.

The fiber reinforcement is not particularly limited and specificexamples include carbon fibers, glass fibers, aramid fibers, boronfibers, silica fibers, and the like. These fibers may be any ofcontinuous fibers and discontinuous fibers. These fiber reinforcementsmay be used alone or in combination of two or more kinds, and inparticular the invention has a remarkable effect when a carbon fiber isused.

Another aspect of the invention is a composite material produced byheat-curing a prepreg made by impregnating into a fiber reinforcement aresin composition including components (A) to (D) below as essentialcomponents, wherein component (D) is localized between laminated layersand forms a phase separated structure (invention of claims 8).

(A) 40 to 70 parts by weight of an aromatic bismaleimide expressed byformula [1] above(B) 60 to 30 parts by weight of an alkenylphenol expressed by formula[2] above(C) 1 to 10% by weight of a polyetherimide of a thermoplastic resinbased on the total amount of components (A) and (B) above, and,(D) 15 to 50% by weight of an amorphous polyimide the glass transitiontemperature of which is 200° C. or higher based on the total amount ofcomponents (A) and (B) above.

In particular, it is preferred that a prepreg is, for example, heatcured at about 180° C., and further post-cured at 200° C. or higher andhas a glass transition temperature of a composite material of 200° C. orhigher (invention of claim 9).

A composite material can be obtained from the prepreg obtained by theinvention described in claim 1 by means of usual, various thermal curingmeans and methods. For example, a composite material is typicallyobtained by heating and/or pressurization by means of a heating andcuring oven by use of a die. The heating and/or pressurizing method byheating or a curing oven is not particularly limited and the examplesinclude methods such as by molding with a usual autoclave, hot pressmolding, and molding by use of a heat-curing oven. The moldingconditions suitably include a pressure of 0.05 to 4 MPa, a temperatureof 80 to 200° C. and a time of 1 to 3 hours. After the prepreg is heatcured, the die is cooled and then a molded product is demolded and takenout.

In particular, a prepreg is used in which the amorphous polyimide ofcomponent (D) obtained in the invention described in claims 4 to 6 isdistributed in the surface vicinity of one or both sides of the prepregto thereby obtain a composite material in which component (D) islocalized between laminated layers to form a phase separated structurewhen a composite material was formed. This composite material isparticularly excellent in toughness.

EXAMPLE

Hereinafter, the present invention will be described in more detail byway of example. The part means the weight part.

Examples 1 to 4

57 Parts of 4,4′-bismaleimide diphenylmethane (Matrimid 5292A: made byHuntsman Corp.), 43 parts of O,O′-diallyl bisphenol A (Matrimid 5292B:made by the Huntsman Corp.) and 2 to 5 parts of a polyetherimide (Ultem1000: made by GE Corp.) were blended at 130° C. for 60 min to dissolvethem and prepare resin component (1). This resin component (1) waskneaded with 20 to 30 parts of an amorphous polyimide particulateshaving an average particle diameter of 10 micrometers (AURUM PD450M:made by Mitsui Chemicals, Inc.) to thereby uniformly disperse polyimideparticulates and prepare resin component (2).

This resin component (2) was impregnated into a reinforced fiber inwhich strands of 410 tex (g/1000 m) of a high-strength and intermediateelastic carbon fiber (made by Toho Tenax Co., Ltd.: IM-600-12K) arealigned using prepreg-making device to produce a unidirectional prepreg.The mass per unit area of the carbon fiber of the prepreg was 145 g/m²and the resin content was 35% by weight. This prepreg was cut out to agiven size and were laminated to [+45°/0°/−45°/90°] 4-s and theresulting materials were molded by autoclave molding at 180° C. for 3hours at 5 atmospheric pressure. The molded material was demolded andthen post-cured for 6 hr in an oven at 200° C. to thereby produce testpieces for compression strength after impact measurement. Thecompression strength after an impact of 1500 in-lb/in was determined inaccordance with SRM 2 of SACMA using this test piece. The results wereshown in Table 1.

Each of the prepregs obtained in Examples 1 to 4 above was 14-layerlaminated in one direction and the resulting material was molded byautoclave molding at 180° C. for 3 hours at 5 atmospheric pressure. Themolded material was demolded and then post-cured for 6 hr in an oven at200° C. and the interlaminar shear strength was determined in accordancewith SRM 8 of SACMA (ILSS). In addition, the interlaminar shear strengthafter the material had been immersed in a methyl ethyl ketone solutionat 23° C. for 6 days was measured. The values of ILSS before and afterimmersion in a methyl ethyl ketone solution were shown in Table 1.

The glass transition temperature of each of the unidirectional moldedsheets obtained in Examples 1 to 4 above was determined at a frequencyof 1 kHz and a rate of temperature rise of 3° C./min in accordance withthe bending mode of JIS K-7244. The peak of loss elastic modulus (E″)was taken as the glass transition temperature. The results were shown inTable 1.

TABLE 1 Example Example Example Example 1 2 3 4 Matrimid 5292 A 57 57 5757 Matrimid 5292 B 43 43 43 43 Polyetherimide 2 2 5 2 Amorphous 25 30 3020 polyimide Compression 335 303 300 260 strength after impact (MPa)Glass transition 235 234 234 235 temperature (° C.) ILSS (MPa) before120 121 120 121 immersion ILSS (MPa) MEK 120 120 119 121 after immersion

Comparative Example 1

A test piece was fabricated as in Example 1 except that a resincomponent was prepared without addition of an amorphous polyimide (AURUMPD450M: made by Mitsui Chemicals, Inc.) and evaluated as in the case ofthe examples. The results were shown in Table 2.

Comparative Example 2

A test piece was fabricated as in Example 1 except that a crystallinepolyimide (AURUM PD250: made by Mitsui Chemicals, Inc.) was used insteadof the amorphous polyimide (AURUM PD450M: made by Mitsui Chemicals,Inc.) and evaluated as in the case of the examples. The results wereshown in Table 2.

Comparative Examples 3 to 4

A test piece was fabricated as in Example 1 except that resin component(1) was prepared without addition of polyetherimide (Ultem 1000: made byGE Corp.) and evaluated as in the case of the examples. The results wereshown in Table 2.

Comparative Example 5

A test piece was fabricated as in Example 1 except that two parts of apolyether sulfone was used instead of polyetherimide (Ultem 1000: madeby GE Corp.) and evaluated as in the case of the examples. The resultswere shown in Table 2.

Comparative Example 6

A test piece was fabricated as in Example 1 except that polyetherimideparticulates (Ultem 1000: made by GE Corp.) was used instead of theamorphous polyimide (AURUM PD450M: made by Mitsui Chemicals, Inc.) andevaluated as in the case of the examples. The results were shown inTable 2.

TABLE 2 Comparative Comparative Comparative Comparative ComparativeComparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Matrimid 57 57 57 57 57 57 5292 A Matrimid 43 43 43 43 43 43 5292 BPolyetherimide 2 2 — — — 30 Polyether — — — — 2 — sulfone Amorphous — —25 30 30 — polyimide crystalline — 30 — — — — polyimide Compression 150127 200 220 175 290 strength after impact (MPa) Glass 236 235 234 234232 233 transition temperature (° C.) ILSS (MPa) 122 121 121 121 120 120before immersion ILSS (MPa) 122 120 120 121 120 100 MEK after immersion

As shown in Table 1, the test pieces that used the prepregs in Examples1 to 4 were high in compression strength after impact and excellent inimpact resistance and toughness. However, the test pieces of ComparativeExamples 1 to 6 that are outside the scope of the invention posedproblems such as low compression strengths after impact. That is, forthe resin compositions not containing an amorphous polyimide(Comparative Examples 1 and 2), or the resin compositions not containinga polyetherimide (Comparative Examples 3 to 5), the compressionstrengths after impact were small and the impact resistance andtoughness were inferior. In particular, with the resin composition inwhich thermoplastic resin particulates other than an amorphous polyimidewere dispersed, the compression strength after impact was greatlylowered, which adversely affected the impact resistance (ComparativeExample 2). For the glass transition point, a decrease in Tg due to theaddition of a thermoplastic resin was not observed. In addition, when apolyetherimide was used in a large amount, the compression strengthafter impact was high, but the solvent-resistant properties (resistanceto methyl ethyl ketone) were poor (Comparative Example 6).

1. A prepreg produced by impregnating a resin composition includingcomponents (A) to (D) as essential components into a fiberreinforcement. (A) 40 to 70 parts by weight of an aromatic bismaleimideexpressed by formula [1], (B) 60 to 30 parts by weight of analkenylphenol expressed by formula [2], (C) 1 to 10% by weight of apolyetherimide of a thermoplastic resin based on the total amount ofcomponents (A) and (B) and, (D) 15 to 50% by weight of an amorphouspolyimide the glass transition temperature of which is 200° C. or higherbased on the total amount of components (A) and (B).

wherein, X is —CH₂—, —C(CH₃)₂—, —SO₂—, —SO—, —CO—, —S— and —O—.

wherein, R₁ or R₂ is each independently an allyl group, n is an integerof 1 to 4, and Y is —CH₂—, or —C(CH₃)₂—.
 2. The prepreg according toclaim 1, wherein the repeated structural unit expressed by formula [3]below and the repeated structural unit expressed by formula [4] below,of the amorphous polyimide of component (D), are 40% by mole or more and5 to 60% by mole, respectively.

wherein, R₃ is a quadrivalent group selected from the group consistingof a monocyclic aromatic group, a condensed polycyclic aromatic groupand non-condensed polycyclic aromatic groups produced by linkingdirectly or via a cross-linked member an aromatic group to each other.3. The prepreg according to claim 1 or 2, wherein the amorphouspolyimide of component (D) comprises particulates and its particlediameter is 100 μm or less.
 4. The prepreg according to claim 3, whereinthe amorphous polyimide particulates of component (D) are localized anddistributed in the surface vicinity on one or both sides of the prepreg.5. The prepreg according to claim 1 or 2, wherein the amorphouspolyimide of component (D) comprises a fibrous nonwoven fabric and thefibrous nonwoven fabric is placed in the surface vicinity of one or bothsides of the prepreg.
 6. The prepreg according to claim 1 or 2, whereinthe amorphous polyimide of component (D) comprises particulates and afibrous nonwoven fabric and the particulates and the fibrous nonwovenfabric are placed in the surface vicinity of one or both sides of theprepreg.
 7. The prepreg according to claim 1 or 2, wherein thepolyetherimide of component (C) is a polyetherimide having a repeatedstructural unit expressed by formula [5] below and a unit number averagemolecular weight of 3,000 to 50,000.


8. A composite material produced by laminating a plurality of prepregsmade by impregnating into a fiber reinforcement a resin compositionincluding components (A) to (D) as essential components, and heat-curingthe laminate, wherein component (D) is localized between laminatedlayers and forms a phase separated structure. (A) 40 to 70 parts byweight of an aromatic bismaleimide expressed by formula [1], (B) 60 to30 parts by weight of an alkenylphenol expressed by formula [2], (C) 1to 10% by weight of a polyetherimide of a thermoplastic resin based onthe total amount of components (A) and (B), and (D) 15 to 50% by weightof an amorphous polyimide the glass transition temperature of which is200° C. or higher based on the total amount of components (A) and (B).

wherein, X is —CH₂—, —C(CH₃)₂—, —SO₂—, —SO—, —CO—, —S— and —O—.

wherein, R₁ or R₂ is each independently an allyl group, n is an integerof 1 to 4, and Y is —CH₂—, or —C(CH₃)₂—.
 9. The composite materialaccording to claim 8, wherein the plurally laminated prepreg isheat-cured and then further post-cured at 200° C. or higher and theglass transition temperature of the composite material is made to be220° C. or higher.